In many gas burner applications it is desirable or necessary to ensure that the ignition device is fully operable before gas is allowed to flow within the system. In essence, the gas ignition device is "proven" prior to gas being allowed to flow through the system. Various approaches have been taken in order to "prove" the ignition device prior to gas flow. For example, one approach requires a visual recognition or detection of the spark from a sparking ignition device prior to allowing gas flow. It has been found that the detection of such a spark is very difficult in the presence of ambient light, and the detection means must therefore be shielded from external light. Another approach is based on the acoustic recognition, rather than the visual recognition, of the spark. Here again, it has been found that it is very difficult to shield against external noise and the detection source must be capable of detecting the particular sound of the spark. Still another approach is based on proving the existence of energy pulses in the spark generating circuit. This approach has inherent problems since it is possible to have such pulses without an actual spark. Still another approach is based upon measuring the electrical resistance of a heater-type ignition device and comparing same to a reference resistance. In this case, the gas valve is not allowed to open until the resistance of the ignition device approximates that of the reference resistance. It has been found that this reference comparing technique requires complex circuitry which is subject to failure and has inherent problems caused by aging of the reference resistance and/or other circuit components. Thus, each of the prior art approaches of ensuring that the ignition device is fully operable before gas is allowed to flow in the system has some inherent problems.
Because of the foregoing limitations and problems associated with the prior art approaches of ensuring that the ignition device is fully operable before the gas valve is allowed to open, it has become desirable to develop a simple, fail safe ignition system which prevents gas flow to the burner until the heater-type ignition device has reached ignition temperature.